Many different processes are known for coating various types of moving flexible webs such as plastic films, paper, etc. Moving webs, freshly coated on one side with one of a variety of coating compositions or dispersions, are extremely sensitive to surface and other defects, particularly defects caused by the flowing of the coating prior to setting by chilling or drying with impinging cool or warm air to harden the coating sufficiently. Any defect present in a coating prior to setting is undesirable because the defect may remain in the final product. Such defects are particularly serious when the coating on the web is one of more gelatino silver halide emulsion layers.
Many modern coaters use air floatation devices to set/dry and transport a web from its dispensing mechanism to its receiving mechanism. One common so-called floatation device is a "Hi-Float" system commercially available from TEC Systems, a division of W. R. Grace & Co. The Hi-Float system use alternating uniformly spaced air bars or push-type gas discharge devices on each side of the web for support. This is illustrated in accompanying FIG. 1. Each such device is used to push the web away by means of impinging gas and the moving web thereby floats through the system in a wave form which is a repeating single wave form. By this described system gas is blown on both sides of the web to balance it between upper and lower nozzles, and although much lower gas velocities can be used to set/dry the web coating than in normal drying zones, some coating defects may be observed. One common defect of this described system is the formation of scratches in the web caused by dragging of either side of the web on the air bars as a result of tension upset and/or unbalanced discharge gas between the air bars on the opposite sides of the web. Close clearance air bars often get a buildup of coating material on them which subsequently causes rubs on the web. Another common defect, particularly when coating photographic silver halide emulsion, is "mottle" which is coating nonuniformity shown as spots or blotches in the web coating. It can be a result of the web coating being disturbed by direct gas impingement before the coating is set. Such defects cannot be tolerated and the defective areas either must be removed or in the event that "perfect" quality film is desired for critical end use markets, such as in industrial x-ray film, the entire film is scrapped.
In an attempt to overcome the disadvantages described above wherein gas is impinging on both sides of the film, it is known to use a series of floatation devices, called air foils or draw-type gas discharge devices, on the side of the web opposite to that bearing a fresh coating. This is illustrated in accompanying FIG. 2. Mottle thereby is effectively eliminated, but it has been observed that with this arrangement of floatation devices, due to the fact that the web is continuously being pulled toward the air foils and/or the fact, that in many coating applications, the web has a tendency to curl towards the side of the web opposite to the side bearing a fresh coating, dragging of the side of the web on the air foils can occur causing scratches and rubs in the web. Dragging is the result of the very close clearance between the air foil and the web. The scratch and rub defect areas of the web must either be removed or the film scrapped depending on the particular use for the coated web.
It is desirable that the above disadvantages be overcome and that a substantially defect-free coated flexible web be prepared by setting/drying under floating or contact-free and substantially flutter-free condition.